A polymer electrolyte fuel cell has an electrolyte membrane that is made of a solid polymer having proton conductivity. Diverse techniques have been proposed with regard to the polymer electrolyte fuel cell. In the polymer electrolyte fuel cell, during each cycle of power generation, a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas are supplied as reactive gases of an electrochemical reaction to a fuel electrode or anode and to an oxygen electrode or cathode, which are formed as catalyst electrode layers on two opposed faces of the electrolyte membrane. Water is produced on the cathode as the product of the electrochemical reaction. The electrolyte membrane is thus kept in the wet state by the produced water during the cycle of power generation. In the polymer electrolyte fuel cell, during an intermittent rest time period, however, no water is produced on the cathode by the electrochemical reaction, so that the electrolyte membrane is dried. One proposed structure of the polymer electrolyte fuel cell is disclosed in Japanese Patent Laid-Open Gazette No. 2003-68318.
In the conventional polymer electrolyte fuel cell, repeated cycles of power generation with intermittent rest time periods frequently change the humidity state of the electrolyte membrane between the wet state and the dried state. The frequent change in humidity state causes frequent expansion and contraction of the electrolyte membrane. The frequent expansion and contraction applies a significant stress (strain) onto the electrolyte membrane and leads to untimely deterioration of the electrolyte membrane.